Assuming that the total average of harvest indexes is 0.4, the following amount of dry matters must be produced for 1,870 million ton of grain production.
Total Dry Matter Production = 2.5 x 1,870 = 4,675 million ton
If the aforementioned plant community's demand for water (600t of water per ton of dry matters) were applied, 2,805 billion ton of water resources would be used. This means that about 400 mm of water resources are needed for each crop's farming period.
Although this amount is supplied from rainfalls during the period and conveyance from other watersheds, the irrigated farmland area is merely 16% of the total farmland for grain production, as mentioned above.
Table 5 is the scenarios of food for 2050 proposed by Kendall and Pimentel (1994). Increase in the farmland area in them appears quite difficult, as only a 20% increase is estimated even in the optimistic scenario. In the sensible scenario, a 12% decrease is anticipated. Based on the scenarios, the water demand of farmlands is estimated as follows. The figures suggest that the amount of water required to secure sufficient amount of food in the middle of next century, is 30% (pessimistic scenario) to 70% (sensible scenario) more than the current demand for water by the farmland for grain production (400 mm/ha).
Pessimistic scenario： 3.6 trillion ton
Sensible scenario： 4.2 trillion ton
Optimistic scenario： 4.8 trillion ton
Nonetheless, in view of the population expansion, upgrading of living standard, and artificial global warming, we can anticipate the following situations that will add great difficulties to securing water resources for agricultural production.
1] Increase in variability of rainfall status associated with global warming. (More shower-type rainfalls, longer dry spells, lower utilization rate of rainfalls, etc.)
